An optical transmitter may use a variety of modulation techniques to modulate an optical signal to carry information. Polarization multiplexing (PM) is a modulation technique where two independent electrical data signals are modulated onto an optical carrier wave having orthogonal polarizations (e.g., an X channel polarization and a Y channel polarization) so that the overall data throughput is doubled without doubling the spectral bandwidth. Quadrature amplitude modulation (QAM) is a modulation technique where two or more binary or multi-level electrical data signals are modulated, via an in-phase, or “I” channel, and a quadrature (90 degree) phase, or “Q” channel, onto a single optical carrier wave such that both the amplitude and the phase of the optical carrier wave are modulated with data to enhance the efficiency of the spectral occupancy. An optical modulator may combine two different signals, each with an I channel and a Q channel, into a single signal with an XI channel, an XQ channel, a YI channel and a YQ channel, thereby improving spectral efficiency relative to a single signal.